Hodge

2012·0.25 MB·English

Erratum

At section 7, second para, line 8 'extant' should read 'extinct'.

Reproduced with the permission of editor Michael Matthews and Springer Netherlands.
Sci&Educ
DOI10.1007/s11191-012-9544-7
Darwin’s Book: On the Origin of Species
Jonathan Hodge
(cid:2)SpringerScience+BusinessMediaDordrecht2012
Abstract This essay is an interpretation of Charles Darwin’s Origin of Species. It
focuses on the contents of the Origin as Darwin intended them to be understood and the
background to the work, thus revealing the originality (or otherwise) of the work.
1 A Composition and a Commodity
When the Origin first appeared late in November, 1859, it was business as usual for one
man:JohnMurray,thebook’sprominentLondonpublisheronAlbemarleStreet.Itspaper,
ink, cloth and board could constitute a book because its composition, production, mar-
keting and consumption made it that kind of commodity; and it could be the particular
commodity it was because a highly reputable man of science had composed it
ThisessayiswrittenagainstthebackgroundofahugeamountofworkonCharlesDarwin—hislife,his
science,hispredecessors,hiscontemporariesandtheirculture,thesuccessorsandthefateoftheideas,and
muchmore.Necessarilythereforemuchispresupposedinthisessay.Tostartintothispertinentliterature,
beginwiththeOriginitself,thefirstedition(1859),easilyavailableonlineorasaninexpensiveboundbook
(thebestisthefacsimileedition,withanintroductionbyErnstMayr,publishedbyHarvardUniversity
Press).ThebestwaytomakeastartoninterpretiveissuesistogotothetwoCambridgeCompanions:one
ontheOrigin(RuseandRichards2008),andtheotheronDarwin(HodgeandRadick2009).Documentation
formuchofwhatissaidhereaboutDarwinandhisworkcanbefoundintwovolumes:Hodge(2008a,b).
TheDewey-MayrthesisisdiscussedatgreaterlengthinHodgeandRadick(2009).Capitalistcontextsare
exploredmorefullyinHodge(2009).ThezoologistDavidReznick’s(2009)bookontheOriginnow
providesthemostdetailedguidetoitsfourteenchaptersandtotheirconnectionswithcurrentevolutionary
biology.TosamplerecentstudiesbymanyspecialistswritingonthereceptionofDarwin’sideas,seeEngels
andGlick(2009).TheCambridgeEncyclopediaofDarwinandEvolution,editedbyMichaelRuse,contains
manypertinentessaysandarticles,includingonebytheauthorontheoriginsoftheOrigininDarwin’searly
notebooktheorising(Ruse2012a).ThisencyclopediaandthetwoCambridgeCompanionsprovidefurther
guidancetothesecondaryliterature.Theydirectonealsotoindispensablewebsites,mostnotablythe
DarwinOnlinewebsitemanagedbyJohnvanWyhe.
J.Hodge(&)
UniversityofLeeds,LeedsLS29JT,UK
e-mail:[email protected]
123
J.Hodge
appropriately for a particular literary genre: an authoritative, but also innovative and
controversial, work of science commercially published (Kohler and Kohler 2009).
Murray and Darwin’s collaboration, though unexceptional at the time, was made pos-
siblebyrecenttechnicalandeconomicdevelopmentsinthemakingandtradingofbooks,
developmentsrespondingandcontributingtochangesinpeoples’employment,leisureand
aspirations and so in society itself. Darwin had earlier followed Lyell—they were both
Whigs—inpublishingwiththeMurrays,aToryScottishfamilydynasty.Since1845,inits
less expensive Colonial and Home Library series, the firm had been doing well with the
secondeditionofDarwin’sJournalofResearches,hisfarmoreinvitingandreadable,and
far less controversial, Beagle voyage book, priced at about a week’s pay for a laboring
man,andinitiallyalsopurchasableinthree,monthlypartsbilledascheapliteratureforall
social classes. At the wholesaling of the Origin, George Mudie bought for his national
chainofcommerciallendinglibraries500copiesofthe1,250printed,atransactionwithno
precedents half a century before. Materially very respectable but not luxurious, each
volumeretailedforaroundhalfthewagesmanymenandmostwomenearnedinamonth.
Boundinwerethirtytwopageslisting,withprices,severalhundredMurraypublications.
There was a mismatch between Murray and Mudie’s aims and expectations and Dar-
win’s hopes and fears. Darwin cared little for the lending library readers, much more for
whatthebookwoulddoforhisstandingandtheacceptanceofhisviewsamonghispeers
in British and foreign scientific elites. He even flattered his closest scientific friends—
Lyell,Huxley andHooker,inhis eyes thenation’s most eminentgeologist,zoologistand
botanist—by claiming to be content if they alone were initially won over to his theories,
since their public support would eventually get decisive numbers of others to give him a
hearingathomeandabroad.TheOrigindescendedindirectlyfromatextualancestor,the
essayofovertwohundredmanuscriptpagesthatDarwinhadsaltedawayin1844inafair
copytobepublishedwerehetodiebeforecompletingamoreextensiveversionofit;and
this essay had been adapted even more closely to winning over a handful of special
scientific associates (Darwin 1909).
What matteredmuch more toMurray and Mudie was what Darwin had had no part in
initiatingordirectingtohisownends:adecadeandahalfofsensationalsalesandmassive
influenceondiscussion,inpopularforumsaswellaselitecircles,ofashorterandcheaper
bookthanDarwin’sOrigin.Itwasabookfirstissuedanonymously,byaleadingLondon
medical publishing house, that same year, 1844. Written by the Edinburgh publisher and
journalist Robert Chambers, Vestiges of the Natural History of Creation was a book
judged, not only by its few ardent supporters but also by many of its more numerous
vehement opponents, to offer a very accessible, engaging account of lawful progressive
development throughout nature and society, from nebulae condensing into stars in the
heavens to men descending from apes here on earth. Predictably, the Origin, although
written as a successor to Darwin’s essay of 1844, was often read as a sequel to Vestiges;
and any accounts of Darwin’s book as at once composition and commodity must com-
prehend such complications (Secord 2000).
Historiansofsciencearetrainedandpaidtoreplacesimplestories,sotheycherishthese
complications. I am contributing to this issue of this journal as a historian of science
specificallyinvitedtowriteasone.InmyconcludingremarksIdiscusswhyhistoriansof
science write as they do, and not as scientists and philosophers tend to. Historians of
scienceareobviouslyexpectedtostudymanydifferentaspectsofanybookasinstructive
andinfluentialastheOrigin.Whatcomenextinthisessayareattemptstorelatethebook’s
content to its structure so that its composition can be related to its contexts.
123
OriginofSpecies
2 The Descent of the Origin
TherearecomplexitiesinthedescentoftheOriginfromitstextualantecedents.Between
theessayof1844andtheOrigincamewhatDarwincalledhisbigbook,NaturalSelection
(Darwin1975).Thisbookwasthevastprojectedtreatisehestartedwritingin1856andquit
workingoninsummer1858—neverreturningtothetask—whenWallace’sletter,withhis
handwrittenessayenclosed,promptedDarwintocomposeanabstract ofthebigbookfor
speedy publication. The abstract was initially intended to be a few dozen pages long but
quicklygrewintothehalf-a-thousandthatappearedinNovember1859.Theessayof1844
haditselfanantecedent,asketchofthreedozenmanuscriptpagesroughedoutin1842;and
thissketchhadexpoundedtheoriesandargumentsfirstarrivedatinnotebookentriesover
the months from spring 1837 to summer 1839.
LyellusedtoexasperateDarwin,notperhapsdeliberately,bysometimesdescribingthe
Origin as expounding a revision of Jean-Baptiste Lamarck’s views in his 1809 book:
Philosophie Zoologique. WhatLyell did notknow was that this description was ina way
correct, thanks to a sequence of influences that Lyell had himself been responsible for,
albeit unwittingly. Before Darwin had ever read Lamarck’s book, he had absorbed the
fifteen-page synopsis of its teachings in Lyell’s Principles of Geology (1830–1833; the
synopsis appeared in the second volume published in 1832). This account of Lamarck’s
views influenced Darwin as Lamarck’s own version could never have done, because
Lyell’s version was quite different in structure and content from Lamarck’s. A fuller
successionofantecedentsofthe1844essayandsooftheOrigincanbediscernedoncethis
complication is recognised: Lamarck’s own version of his system appears in 1809; a
radicaltransformationofitisexpounded—andrejected—byLyellin1832;Darwinopens
his Notebook B in July 1837 with two dozen pages sketching a system deliberately
matching,initsstructureandcontent,Lyell’sversionofLamarck,butwithconsequential
departures and additions; within days this system is transformed by Darwin into another,
new version involving a new conception of the tree of life (Barrett et al. 1987); this new
versioniseventuallychangeddecisivelywhenthetheoryofnaturalselectionisintegrated
withthisconceptionofthetreeoflife,startinglatein1838andearly1839,andsowhenthe
views later set out in 1842, 1844, 1856 and 1859 are first largely in place.
Allsuchcomplicationsinconceptualandtextualtransformationsarenotonlypertinent
tobiographicalandbibliographicalnarratives;theycanprovideindispensablelightonthe
argumentationoftheOrigin.Thefirstedition’sthirteenchapters,aftertheintroductionand
before the final recapitulating and concluding chapter (XIV), form three clusters: an
openingfourchapters (I-IV)makingthe casefornaturalselection asexistinginthewild,
and able to produce new species from old; next, five chapters (V-IX) addressing supple-
mentary issues and countering difficulties, and then four more chapters (X-XIII) showing
howmanykindsoffactualgeneralisationsaboutspeciesthetheoryofbranchingdescentby
natural selection can explain. Essential to what Darwin calls—in opening his closing
chapter (XIV)—the one long argument of the book is, therefore, the dichotomous divide
between those first four (I-IV) and later four (X-XIII) chapters.
This divide descends from a dichotomous divide in Lyell’s synopsis of Lamarck, a
divide with no precedent in Lamark’s own exposition, but a divide matched in Darwin’s
systemic sketch of July 1837 (Barrett et al. 1987, 180, B: 1–24): a divide between a first
section on adaptive branching species descents going on now, and a second section on
escalating progress, from the simplest organisms to the highest, over eons of the past. In
1838, months before Darwin integrated his tree of life with natural selection, this divide
had been transformed into a successor divide decisive for his plans for the prospective
123
J.Hodge
bookhealreadyhadinmind.Thistransformeddivideisverymuchwhatwillstructurethe
Origin:adividebetweenmaking,first,theevidentialcaseforatheoryaboutthecausesof
species origins in branching descents, a case independent of any facts the theory is to
explain,andthen,second,makingafurtherevidentialcaseforthetheorybydemonstrating
its explanatory virtue concerning many different kinds of facts. With this divide, and its
precedentsinLyell’sPrinciplesandNewton’sPrincipia,theargumentationalexpositionof
the Origin is not back-to-front but the right way round according to traditional norms
(contraSober2010).Therationaleforthisdivideisbestintroducedafterabrieflookatthe
evidential ideals decisive for the whole book.
3 Analogia, a Fortiori and Vera Causa
Darwin’scasefortheadequacy,thecompetenceofnaturalselectionasacauseofspecies
origins in branching adaptive descents invokes an analogy with the form of a traditional
analogy as proportionality: the struggle for existence is to wild animals and plants as a
humanbreederistodomesticones.Withanyanalogicalproportionalitythereisarelational
comparison,hereacomparisonbetweentwocauses:thestruggleandthebreeder.Forthese
two causes have the same relation to the animals and plants, wild and domestic respec-
tively, that they act upon. It is a causal relation: namely, causing hereditarily variant
individuals to differ in their chances of survival and reproduction. Although relationally
alike, the struggle and the breeder are intrinsically, in themselves, very unlike agencies.
The natural selection entailed by the struggle and the artificial selection entailed by the
breeder’spractisesarealikebothrelationallyandintrinsically.Darwin’sreasoningstoand
from the selection analogy depend on these relational and intrinsic comparisons, and on
intrinsiccontraststoo:natural andartificialselectionare thesame kindof causalprocess,
but differ in degree, nature’s selection being vastly more powerful.
Argumentsafortiori,fromthestronger,candeployjustsuchcomparisonsandcontrasts.
Considerthealternationofaproportion,asininferringfromtenbeingtofiveastwoisto
one that ten is to two as five is to one. Darwin argues not only to and from the propor-
tionalitybetweennaturalandartificialselectionandtheirrespectiveeffects,hearguesalso
toandfromitsalternation:naturalistoartificialselectionaswildspeciesformationsareto
domesticvarietyformations.Giventhatnature’sselectionissomuchmorepowerfulthan
man’s,andthatman’sselectioncanproducevarietiesonthefarm,then,afortiori,natural
selection can produce far greater effects, species, in the wild.
Thevera causaevidential idealhadnoinherentconnectionwithanalogicalnorwitha
fortiori reasoning; but this ideal is integrated in Darwin’s Origin with both. A traditional
requirement for a causal-explanatory hypothesis was that the cause it invokes should be
shown to be able to produce the kinds and sizes of effect that it is to explain. Further, a
good hypothesis should be able to explain many different facts about those effects. Such
explanatoryvirtue isevidenceforthe existenceof thiscauseandforitsresponsibilityfor
thoseeffects;but,becausethisevidencefortheexistenceofthecauseisnotindependentof
the facts explained, the hypothesis and the cause are deemed conjectural, speculative,
hypothetical. By contrast, a vera causa is a cause that has its existence also evidenced
independently, by facts other than those it is to explain. So, to show that some causal-
explanatorytheoryisnomerehypothesisbutaveracausatheory,andhenceinductive,not
conjectural, three requirements had to be met: the two met by any good hypothesis—
evidence of causal adequacy and demonstration of explanatory virtue—and a third, the
123
OriginofSpecies
requirementthatthecausebeareal,trueorknowncause,acause,thatis,withitsexistence
evidenced independently of the theory’s explanatory virtue.
This vera causa requirement was defended canonically by the eighteenth-century
Scottish philosopher Thomas Reid in elaborating on a Newtonian dictum. Darwin, fol-
lowing his early commitment to Lyell’s geology, with its claim to be vera causa kosher
endorsedbyJohnHerschel(1830),hadlongseentherequirementasanidealforallgood
theorising in inductive science. In conscious accord with this conviction, in the first four
chaptersoftheOrigin(I-IV)Darwinarguesfortheexistenceofnaturalselection,andfor
itsabilitytoformnewspeciesfromoldinindefinitelyextendedbranchinganddiversifying
descents; while in those later four chapters (X-XIII) he makes the case for branching
natural selection having been responsible for the formation of extant and extinct species.
The case is made by displaying this theory’s ability to explain many factual generalisa-
tions—geographical, embryological and other generalisations—about species.
Darwinhasthenaddressedthreequestionsaboutnaturalselection:Isit?Couldit?Did
it? His selection analogising relates to his three evidential cases—the existence, the ade-
quacyandtheresponsibilitycase—indistinctways.Theanalogydoesnotcontributetothe
existencecase;fornaturalselectionisshowntobeanexistingcause,averacausa,through
arguments from Lyellian geology, Malthusian population dynamics and generalisations
about variation in domestic animals and plants, but not from generalisations about their
selective breeding. The analogy does evidence the power and so the adequacy of natural
selection;andbecausetheanalogyshowswhateffectsnaturalselectioncanhave,itshows
what facts can be explained as its effects in making the responsibility case.
4 The First Four Chapters (I–IV)
Introducing the first (1859) edition of the Origin requires trawling through its chapters—
after a warning and a suggestion: Darwin’s prose is often not very lucid; for Darwinism
expounded more clearly one goes to the books of Alfred Russel Wallace.
The opening chapter (I) on variation in domesticatedspecies discusses variation itself,
andthenvariationasaccumulatedthroughtheartofselectivebreeding;thesecondchapter
(II)concernsvariationbutnotselectioninnature;thethird(III)introducesthestrugglefor
existence in nature, and indicates briefly how it causes in wild species a natural selective
breeding comparable intrinsically and relationally to man’s selection; then the fourth
chapter (IV), devoted to natural selection, argues that due to its powerful cause, the
struggle for existence, natural selection is adequate, powerful enough, to produce over
eons, from common ancestral species, unlimited adaptive, branching divergences and so
unlimitedly diverse descendent species.
Throughoutthesechapterstherearenotmerelyappealstofactualgeneralisationsabout
variationorselectiononthefarmorinthewild;fortheargumentsincludepremisesabout
the causes of these tendencies and processes. One rationale for these causal emphases is
that all four chapters contribute to an explicit contrast: the tendency to variation in
domesticspeciesisgreaterthanthetendencytovariationinthewild;butwiththeselective
breedingsitistheotherwayround:nature’sselectionsovastlyexceedsinpowerman’sas
tomorethancompensateforthelessertendencytovariationinthewild.Variationonthe
farmandinthewildhavetheverysamecauses:changesinconditions—ofsoil,nutrition,
weather and so on—that disrupt sexual and asexual reproductions which would in
unchangingconditionsyieldoffspringexactlyliketheirparents.However,thesecausesof
variationareactiveandeffectivetoahigherdegreeunderdomesticationthaninnature.By
123
J.Hodge
contrast, the causes of selection under domestication and in nature—the human breeder’s
practices and the struggle for life—are entirely unalike, but their selective consequences
for survival and reproduction are the same in kind though not in degree.
The first causal theme in the opening chapter (I) is that in domestic species inherited
variationisabundantlycausedespeciallybyinfluencesontheparents,influencesaffecting
thereproductiveelementspriortoconception,andalsobytheeffectsofhabitsorindeedby
the direct action of changes in diet and the like; while the second causal theme is that
selection by man, rather than crossing or inbreeding, has been by far the main means
wherebythisinheritedvariationhasbeenaccumulatedoversuccessivegenerationssoasto
makedistinctvarietiesorbreedsservingman’susesorfancies.Conspicuouslythereisno
emphasis here on the inherited variation due to preconceptional influences arising by
chance, by, that is, the action of small, hidden, unknown prenatal causes that produce
useless and unwanted as well as useful and wanted variations; but the implication is that
selectionisefficaciousinworkingwithanyinheritedvariationswhetherduetochanceor
not.Thisefficacyisevidentinmethodicalselectionwhenthebreederworksdeliberatelyto
makeavarietytofitparticularneedsorwishes;andevenmoreintheunconsciousselection
resulting from the practice of breeding from the best individuals over many generations,
with no conscious intention of changing the whole breed.
There is one principal causal theme in the second chapter (II), but it is not presented
explicitlythere,onlyintherecapitulationatthebook’send:geology,Lyell’sgeologythat
is,showsthat everyregionhasbeenandstill iscontinuallyundergoingphysical changes,
soanimalsandplantsareatalltimescausedtovaryheritablyundernaturejustastheyare
inchangingconditionsunderdomestication.Thesecondchapteritselfarguesthatspecies
inlargergenerausuallyhavemorevarietiesthanspeciesinsmallergenera,becausethere
has been more variability in wider ranging groups exposed to more varied conditions.
Here,Darwininterpretsvarietiesasincipientspecies,andspeciesaswell-markedvarieties
differing in degree but not in kind from varieties. The third chapter (III) emphasises that
there is always in the wild a competitive struggle to survive and reproduce owing to the
tendencyofallspeciestoincreasetheirnumbers,andtothecheckstothoseincreasesfrom
limitations on food and other resources. The principal causal theme of the fourth chapter
(IV), on natural selection, is anticipated here when this struggle is cited as the cause
accumulating hereditary variation selectively and so adaptively.
Thefourthchapter itselfstartswiththe greaterpower of nature’soverman’sselection
due to nature’s being more prolonged, more precise and more comprehensive; nature
selectingovereonsamongallthoseveryslightvariationsmakingforsmall,butinthelong
rundecisive,differencesinchancesofsuccessorfailureinthecompetitiontosurviveand
reproduce. Complementing this natural selection in competition for survival and repro-
duction,thereissexualselectionincompetitioninwinningmatesthroughmalecombator
female choice, with arms (stags’ antlers) or charms (peacocks’ tails). In the middle five
chapters, sexual selection will be integrated with generalisations about variation in sec-
ondarysexualcharacters;andwillbeenhancingthecausaladequacyofselectiongenerally,
especially in causing features too disadvantageous in the struggle for life to be due to
naturalselection.However,inthelaterclusteroffourchapters(X-XIII),Darwinwillfind
no explanatory work for sexual selection. This complement to the theory of natural
selection,asitisintheOrigin,hastowaituntil1871andtheDescentofMantocomefully
and publicly into its own as an explanatory resource.
Theendofthechapter(IV)isdominatedbytheprincipleofdivergence:structuraland
functional specialisation is usually advantageous in life’s struggle; so over eons natural
selectioncauses,reliablyifnotinvariably,structuralandfunctionaldivergencesbytending
123
OriginofSpecies
to favor diverse adaptive specialisations; so causing diversifying, branching descents
among the more specialised winning species and terminal extinctions among the less
specialised losers. Since increased structural and functional differentiation in animal and
plant organisation constitutes progress, natural selection as a reliable cause of adaptive
change is no less a reliable, if not invariable, cause of progressive change.
5 Three General Evidential Considerations
Writing on the 22nd of May 1863 to George Bentham, the eminent botanist and author
when young of a book on logic, Darwin insisted that:
thebeliefinnaturalselectionmustatpresentbegroundedentirelyongeneralconsiderations.(1)on
itsbeingaveracausa,fromthestruggleforexistence;&thecertaingeologicalfactthatspeciesdo
somehow change (2) from the analogy of change under domestication by man’s selection. (3) &
chiefly from this view connecting under an intelligible point of view a host of facts.—(Letter to
GeorgeBentham,22May1863,Darwin1985,11:433)
Bycontrast,hesays,withthesegeneralconsiderations,whenwedescendtodetailswecan
confirmofnoonespeciesthatithaschanged,northatthesupposedchangesarebeneficial,
nor why some species have changed while others have not. This contrast between the
general and the particular evidences may seem to concede more than Darwin does in the
argumentoftheOrigin,whichisafterallfullofdetailsaboutparticularspeciesinthewild
and varieties in gardens, on farms and in pigeon lofts. However, the book’s argument
always works within this concession, in that all such details are in support of general
grounds for accepting the theoretical theses, rather than providing direct testimony as
observed instances of branching descent or of natural selection in action.
In Darwin’s three general considerations, the invocation of geology in the first one is
cryptic. Once again, what Darwin needed to say here is that geology shows that wild
species live in continually changing conditions causing inherited variation. It is this cau-
sation,togetherwiththestruggleforlife,which,heholds,entailstheexistenceofnatural
selection in the wild. The second general consideration is the analogical comparing and
contrastingofnature’sselectionwithman’sinestablishingwhatnature’sselectioncando
initsmuchlongerrun.SomuchthenfortheOrigin’sfirstfourchapters.Thethirdgeneral
considerationreferstowhatisexpoundedinthelaterfourchaptersongeology,geography,
morphology and the rest.
InenlighteningBentham,Darwindoesnotcitetheselectionanalogyasestablishingthat
natural selection is a vera causa, an existing cause; and quite rightly (Ruse 1975 is
mistakeninthis).Inhisnotebookswhenfirstarrivingathistheoryofnaturalselectionand
in the Origin, Darwin does not do what it is easy but mistaken to assume he must have
done: namely, to argue that domestic varieties are known to be produced by man’s
selectiononthefarm,thatspeciesarelikedomesticvarieties,so,thereforethereexistsin
the wild a similar natural process of selection which produces species. Rather, in the
notebooksandin1859,theargumentfortheexistenceofselectioninnatureappealstothe
existenceinthewildofinheritedvariation,andtotheexistenceofastruggleforlifeacting
discriminatinglyonthatvariation;sothereexistsaprocessofcumulativediscriminationof
variationinnature;andthisprocessislikeselectivebreedingonthefarmbutmuchmore
powerful. What is known about man’s selection and about its relational and intrinsic
likenesstonaturalselectiondoesnotestablishthatnaturalselectionisanexistingcause,a
vera causa; but it does indicate, as in Darwin’s second general consideration, what this
natural process can produce.
123
J.Hodge
6 On the Middle Five Chapters (V–IX)
Darwin’s three general evidential considerations do not map onto the Origin’s three
clusters of chapters. The first of the middle five chapters (V), on laws of variation, sup-
plements the book’s two opening chapters (I, II) on variation under domestication and in
nature.Themiddlethreeofthemiddlefive(VI-VIII)thencounterobjectionstothethesis
of the book’s fourth chapter: that natural selection is competent, adequate to cause the
formationandadaptivediversificationofspecies.Thelastofthemiddlefive(IX)belongs
withthelaterfourchapters,thechaptersdisplayingthetheory’sexplanatoryvirtue,andso
providing the evidence for natural selection having been responsible for producing the
extantspecieslivingtodayandtheextinctspeciescommemoratedasfossils.For,thisfifth
chapterofthemiddlefivetakescareinadvanceofanobjectiontothatresponsibilitycase;
and does so by insisting that the fossil record is not a complete and reliable record of
sporadic, sudden, jumpy exchanges of new species for old; rather, it is a patchy, gappy,
intermittent, damaged, fragmentary record of what were gradual transitional changes in
speciesandintheirconditionsoflife.So,properlyunderstood,thefossilrecordpresentsno
insuperabledifficultiesfortheviewthatthosechangeswereslowly,smoothlywroughtin
gradual branching descents by means of natural selection.
Opening that middle miscellany of five chapters, the chapter on laws of variation
(V) begins with Darwin saying that he has sometimes spoken of variations being due to
chance, but that this expression is improper in implying that all variations are not due to
lawfulcauses.Infacthehastalkedonlyonceortwiceinpassingofchancevariations,and
thelargeraimofthechapterisnotlimitedtocorrectinganymisleadingimpressionssuch
talk might have given. For what the chapter mainly argues for is a unification thesis.
Variations in domestic and wild plants and animals all conform to the same laws. For
instance,organsdevelopedtoanextremedegreeinsomeorganismswillbeveryvariable
intheirmorenormalcloserelations.Again,thelawsofvariationarethesameforspecies
as for varieties; structures varying between species vary similarly within species. So,this
chapter contributes to the thesis, of the book’s second chapter on variation in nature (II),
that species and varieties differ in degree but not in kind, species being well-marked
varieties and varieties incipient species.
After thischapteron the lawsof variation comesachapter (VI)often anticipatingand
countering various reasons for thinking natural selection incapable of producing new
species from old, because some species have features that selection can not produce,
especially such organs of extreme complexity and perfection as the eye. Darwin’s coun-
teringofthisdifficultyappealstotheexistencetodayofagraduatedarrayofusefulorgans
fromtheeyeondowntosimplestructuresconferringmeresensitivitytolight,anarraythat
makesitconceivablehoweyescouldbeproducedgraduallyovereonsbynaturalselection.
The following chapter (VII) takes on equivalent challenges presented by complexity and
perfection in instincts such as honey bees exhibit in building their combs; and the same
countering strategy is deployed here. In explaining how sterile neuter insects could owe
their instincts to natural selection when they do not breed over successive generations,
Darwinarguesthatifselectionisadmittedtotakeplaceamongfamiliesaswellasamong
individuals the difficulty can be overcome. He implies that this admission entails no
significant amendment to the theory of natural selection, as farmers have likewise
improved the quality of castrated steers by consistently breeding from the parents of the
best.
Thenextchapter(VIII)confrontstheobjectionsthatspeciesarequiteunlikevarietiesin
theirinabilitywhencrossedtoproducefertilehybridoffspring;andthatthisintersterilityof
123
OriginofSpecies
speciespermanentlyensuresthedistinctnessofspecies,andisnotafeatureofspeciesthat
naturalselectioncouldproduce.Darwindisputestheviewthatallandonlyspeciesandno
varieties are intersterile; for intersterility is not always either completely present or
completelyabsentinspeciesorinvarietiesbutcomesindegrees;andhearguesthat,while
not directly due to the action of natural selection it can be indirectly so, because it is a
gradual, incidental consequence of those adaptive divergences in hereditary constitutions
produced by natural selection over many generations. Here too, then, no amendment is
needed to the theory expounded in the book’s first four chapters.
7 On Those Later Four Chapters Preceding the Last One (X–XIII)
Theselaterfourchapterstakeupthreeclustersoftopics;forthefirst(X)isongeology—
which,recall,hasjusthadthelastofthemiddlefivedevotedtoit—thenexttwo(XI,XII)
tobiogeographyandthefourth(XIII)totaxonomyandmorphologyincludingcomparative
embryology.Thesethreeclustersmayseemdistinctenoughthatnounifyingthemescanbe
seen running through them all, except the theme itself of branching descent by means of
natural selection or branching selection for short. However there are revealing, recurrent
themesinDarwin’smanydeploymentsofbranchingselectionasanexplanatoryresource.
ItisworthdwellingonthesethemesasDarwingivesnoclueastowhyhehastreatedthese
threeclustersoftopicsintheorderheadopts.Nordoesheexplainwhyhegivesthethird
cluster far fewer pages than the second, biogeographical cluster, an imbalance without a
precedentintheEssayof1844.Oneistemptedtoguessthatdoingsoallowedhimtofinish
atextthathadalreadytakenmanymoremonthsandmorewordsthanoriginallyplanned.
Consider next an exegetical mistake about the book as a whole (Sober 2010). That
mistakewouldviewitsearlychaptersasconcernedwithnaturalselection,andtheselater
ones as concerned instead with common descent. That this view is mistaken can be
confirmedsimplybynoticingDarwin’srepeatedreferencestonaturalselectionthroughout
these four later chapters. Now, although mistaken, this view can be useful for three rea-
sons. First, Darwin is often appealing to common descent without appealing even
implicitly to natural selection. For instance he discusses the geological generalisation
knownasthelawofthesuccessionoftypes,thelawthattheextantspeciesfoundasfossils
inanyregionareoftensimilarenoughtotheextantspecieslivingthereastobelonginthe
samegeneraorfamilies.Naturally,Darwinexplainsthiscloselikenessasduetodescent,
anddeclaresdescentsupportedevidentiallybyitsyieldingthisexplanation;buthedoesso
without explicitly complementing this declaration by urging that the slight differences
between the extant and extinct species are best explained as due to selection. What such
cases indicate, then, is that Darwin in these chapters does far more explaining of resem-
blances as caused by descent than explaining of differences as caused by selection.
A second reason for that exegetical mistake being useful concerns alternative explan-
atory options. Consider various traditional explanations for structural and functional
resemblances among animals and plants (Russell 1916). Some resemblances might be
explained as due to the organisms being near each other in a scale of organisational
perfection, theassumptionbeing thatonlyonetypeoforganisationispossibleatanyone
level in this scale. Other resemblances might be explained as due to common fittings,
adaptations, providential or otherwise, to common ways of life, aquatic life, say. Again,
both these explanations—common level and common adaptation explanations—might be
rejectedandanexplanationgiveninsteadbyshowingthattheseorganisms,thevertebrate
animalsforexample,areallconstructedonacommonplan(asisarguedbyOwen1849).
123